The present invention relates generally to a cylindrical drum for securing thereto a sheet of film by a partial vacuum or suction. More specifically, the invention to relates to a drum which is adapted to hold film sheets of different sizes for exposure by a controlled light beam during rotation of the drum.
It has been known to prepare plates for printing presses using a photographic technique in which an unexposed film has recorded thereon the desired pattern to be printed. More recently, and for example in the case of printed business forms, computer equipment has been used for composition of the pattern to be printed. Information relating to the desired pattern is edited and then stored within the computer, and finally is used to control exposure of the film for the making of the printing plate.
One component of such a system is the mechanism for exposing the film and, associated therewith, an apparatus for securing the film during exposure. In one known system, the film is exposed using the fine beam produced by a laser. Positioning of the light dot produced on the film by the laser is achieved by securing the film to a rotating drum and directing the beam onto the drum. The dot is moved back and forth along the length of the drum using a movable prism or mirror. Rotational and linear positions of the dot on the drum are monitored and synchronized with blocking and unblocking of the laser to produce the desired image.
It is critical to successful operation of such a system that the film be accurately positioned and securely held on the drum. Otherwise, the laser beam will not be directed to the proper location.
In one present system of this type, a vacuum drum is provided with holes extending through the drum surface near the ends of the drum. A high vacuum pump is connected to the drum to remove air from its interior. The film is placed over the drum, although not over any of the holes and the transparent cover sheet is placed over the film. (A few holes may be provided beneath the film to remove any trapped air.) The cover sheet is tightly secured along its side edges by the vacuum, and the cover sheet in turn secures the film to the drum. It is important that the film be tightly held, as rotational speeds of as much as 2000 rpm are common for equipment of this type. Since this system relies upon a high vacuum with a low volume of air removed, even a very slight leak can result in the film being released from the drum.
Such a system is advantageous in that it enables the drum to secure film sheets of various sizes. This is important, since plates of differing size must be produced. Because the cover sheet is of the same dimensions regardless of the film being secured, all vacuum holes formed in the drum will be covered in every case. Consequently, a very simple drum structure may be used.
However, several problems are presented by the use of a cover sheet to hold the film on the drum. As can be readily appreciated, positioning of the film beneath the cover sheet can be a difficult operation. Moreover, the cover sheet must be fitted to the drum without any wrinkling, as this will affect the optical properties of the cover sheet through which the laser beam must pass to expose the film. Further, the cover sheet is susceptible, particularly during periods of low air humidity, to the build-up of a static charge along the sheet. This in turn attracts dust, which degrades the quality of the image placed on the film. Further, as the cover sheet is removed, the static may be discharged at the separation point between the sheet and the film. Any sparks generated as a result will expose the film.
In addition, the cover sheet may tend to stretch as a result of repeated installation and removal on the drum, thereby requiring periodic trimming or frequent replacement. Further, the sheet must be formed from a material that does not exhibit any significant decrease in transparency over time, and any such change must be uniform throughout the sheet.
It would therefore be of great advantage to design a vacuum drum for securing a film that does not require a cover sheet. To do this, a drum must be provided that is not affected by certain of the vacuum holes in the drum being uncovered, as this will occur whenever a film sheet is used that is of a size smaller than the outer surface of the drum. However, uncovered vacuum holes will tend to lower the vacuum that is produced in the drum interior. Consequently, a number of drums have been designed that rely upon various porting or valving devices to close unused holes. See for example U.S. Pat. No. 4,005,653 to Arkell and U.S. Pat. No. 2,753,181 to Anander.
It has been disclosed in U.S. Pat. No. 3,630,424 to Rau that it is possible to construct a vacuum drum without internal porting or valving that will work in situations where less than the entire drum surface is covered. Rau describes a drum having its surface covered with a plurality of parallel, V-shaped grooves that are interconnected with larger grooves having holes formed therein for communication with the interior of the drum. The smaller grooves are closely spaced and are quite shallow. This drum is described by Rau as working well when used to isolate tension in a web moving around a portion of the drum surface.
What is needed, therefore, is a means for mounting the film for exposure that avoids the problems noted above. Specifically, a vacuum drum is needed that does not require internal porting or valving, but that performs with single sheets of film rotated at high speeds, even in cases where a number of the vacuum holes are left uncovered. Such a drum would enable much more convenient exposure of the film, while retaining the general advantages of a vacuum drum.